When did dinosaurs live (and how do we know)?
Fossils, including dinosaur remains, are found in layers of rock, commonly called strata. The field of stratigraphy is the geological specialty that tells us how old or young particular strata are; thus, stratigraphy is a means of learning the age of dinosaur fossils. Stratigraphy is divided into geochronology or Earth time stratigraphy (geo – Earth; chronos – time), lithostratigraphy or rock stratigraphy (lithos – rock), and biostratigraphy or stratigraphy based upon the presence of fossils (bios – organisms).
Geologists generally signify time in two ways: in numbers of years before present, and by reference to blocks of time with special names (like “Triassic” and “Mesozoic”). For example, we say that the Earth was formed 4.6 billion years before present, meaning that it was formed 4.6 billion years ago and is thus 4.6 billion years old. Unfortunately, determining the precise age in years of a particular rock or fossil is not always easy, or even possible. For this reason, geologists have divided time into blocks of varying lengths, and rocks and fossils can be referred to these blocks of time, depending upon how exactly the age of the rock or fossil can be estimated. For example, you might not know that a fossil was 92.3 million years old, but you might be able to determine that it was within the interval of time known as the Late Cretaceous, meaning that its age is somewhere between 100.5 and 66.1 million years old, dates about which you have more information.
We start our discussion intuitively, by talking about the age in years, or the numerical age. Later we will address the division of time into blocks of varying lengths.
Geochronology: the ages of the ages
Geoscientists are happiest when they can learn the numerical age of a rock or fossil; that is, its age in years before present. Ages in years before present are reckoned from the decay of unstable isotopes (see Appendix 2.1 at the back of this chapter) found in certain minerals. Unstable isotopes spontaneously decay from an energy state that is not stable (that is, that “wants” to change) to one that is more stable (that is, that will not change, but rather remain in its present form).